This invention relates to servo units in optical type information reading devices. More specifically, it relates to a three-beam system focus and tracking servo unit in an optical type video disk information reading device.
FIG. 1(a) is a plan view of a component of an optical type video disk, and FIG. 1(b) is a sectional view of the component. In the optical type video disk, as shown in FIG. 1, pits 2 (recesses) are provided in one surface of a disk substrate made of transparent material in such a manner that they form coaxial or spiral tracks. Data are recorded by the lengths and intervals of these pits 2. The surface including the pits 2 is covered with a reflection film obtained by vacuum-evaporating aluminum in order to increase the optical reflection factor. A protective film 3 is placed on the reflection film.
Reading data out of the disk 1 is carried out as follows: Light is applied to the surface of the disk 1 where no pit 2 is provided and the reflection light from the data record surface 4 which has been modulated by the presence or absence of the pits 2 is demodulated. In such an information record disk reading device, a so-called "focus and tracking servo unit" is provided in order to accurately focus the incident light on the disk data record surface and to position the focussed incident light on the track of the disk at all times.
FIG. 2 is a schematic diagram showing a conventional three-beam system focus and tracking servo unit. Beams emitted from a light source 5 such as a helium neon laser are passed through a collimator lens 6, a beam splitter 7 and a movable mirror 8. They are then focussed in the vicinity of the data record surface 4 of the disk 1 by means of a focussing lens 9. The disk 1 is rotated at high speed by an electric motor 10. The reflection light from the record surface 4 of the disk is passed through the focussing lens 9 and the movable mirror 8 to the beam splitter 7, where it is split and thereafter the resultant beam is converted into an electrical signal by a photo-electric conversion element.
In such systems, it is difficult to make the disk 1 completely flat. Usually, the disk 1 is inclined when placed over the rotary shaft of the motor 10. Accordingly, the record surface 4 is moved up and down as the disk 1 is rotated. In order to correctly read the data out of the disk, the movable mirror 8 and the focussing lens 9 must follow the vertical movement of the record surface and the track of the pits 2 to focus the beams on the record surface 4 and on the track at all times. In order to satisfy this requirement, a cylindrical lens 11 is provided ahead of the focal point of the reflection beam from the disk 1, which is formed by means of the focussing lens 9. After the lens 11 a focussing light receiving element 12 and a pair of tracking light receiving elements 17a and 17bare provided. The light receiving elements 17a and 17bare disposed on both sides of the light receiving element 12. These elements 12, 17aand 17b are positioned linearly in the track direction. The focussing light receiving element 12 is made up of four independent light receiving units 12a, 12b, 12c and 12d as shown in FIG. 3. These light receiving units 12a through 12d are arranged so that lines separating the units from one another form 45.degree. with the central axis of the cylinder of the cylindrical axis. This technique is disclosed in co-pending U.S. patent application Ser. No. 48,421, entitled "Automatic Focus Servomechanism in Optical Information Reading Device", filed on June 14, 1979 and commonly assigned. A position on the optical axis, at which the light beams passing through the lens 11 are focussed, on the plane including the generating lines of the cylindrical lens is different from that on the plane perpendicular to the aforementioned plane. By utilizing this principle, the configuration of light beams applied to the light receiving surfaces of the four light receiving units 12a through 12d are detected and measured to determine the relation between the record surface 4 and the focal point of the focussing lens 9.
More specifically, the light receiving surfaces of the light receiving units are arranged at the position where, when the focal point of the incident light through the focussing lens 9 is correctly positioned on the record surface of the disk 1. This is shown in FIG. 4(a) where the reflection light passing through the cylindrical lens 11 forms substantially a squaure (FIG. 4(b)). Under this condition, the outputs Va, Vb, Vc and Vd of the light receiving units are equal to one another, and the following equation is established: EQU Va+Vb=Vc+Vd
Accordingly, the output V of a differential amplifier 13 (FIG. 3) receiving differential inputs (Va+Vb) and (Vc+Vd) is zero. Therefore, since the output of an amplifier 14 is zero, and the output of a lens drive device 15 is also zero (FIG. 2), and the position of the focussing lens 9 is maintained unchanged.
When the incident light is focussed behind the record surface 4 as shown in FIG. 5(a), i.e., when the distance between the record surface 4 and the focussing lens is shorter, the configuration of the incident light on the light receiving surface of the light receiving element 12 is shown in FIG. 5(b). Therefore, (Va+Vb)&gt;(Vc+Vd), and V&lt;0 (V being the output of the differential amplifier 13).
When the incident light is focussed ahead the record surface as shown in FIG. 6(a), then the configuration of the light on the light receiving surface is as shown in FIG. 6(b). Therefore (Va+Vb)&lt;(Vc+Vd), and V&gt;0.
Thus, the output of the differential amplifier 13 corresponding to the configuration of the light beam on the light receiving surface of the light receiving element is employed as an error signal, which is amplified by the amplifier 14 and is then converted into displacement data by the drive device 15. As a consequence the position of the focussing lens 9 is controlled by a holder 16; that is, automatic focus control is carried out. Furthermore, a tracking drive 20 receives the outputs of the pair of tracking light receiving elements which are provided on both sides of the focussing light receiving element 12 linearly in the track direction of the disk 1 to position the movable mirror 8. Hence, tracking control is carried out to correctly apply the focussed beam to the track of the pits 2.
In the focus and tracking servo unit as described above, when the focussed beam is correctly applied to the track of the pits 2, i.e., when tracking is correctly obtained, the track of the pits 2 is projected as a dark stripe onto a straight line connecting the centers of the light receiving surfaces of the light receiving units 12a and 12b, or 12c and 12d. Accordingly, as long as the proper focalization of the focussing lens is obtained, the output of the differential amplifier 13 receiving the differential inputs (Va+Vb) and (Vc+Vd) is zero. However, if the tracking is incorrect, then the inputs (Va+Vb) and (Vc+Vd) deviate from each other. As a result, an error signal is produced by the light receiving element 12 to reversely affect the focus servo operation.
In order to overcome this difficulty, a method has been provided in which the cylindrical lens 11 is arranged in such a manner that the central axis of the cylinder forms 45.degree. with respect to the track direction of the disk 1. However, this method is still disadvantageous for the following reason. Since one pair of tracking light receiving elements 17a and 17b are provided on both sides of the focussing light receiving element 12 linearly in the track direction of the disk 1, and the central axis of the cylindrical lens forms 45.degree. with the track direction of the disk 1. A pair of tracking controlling light beams passing through the cylindrical lens 11 cannot reach the light receiving surfaces of the paired light receiving elements 17a and 17b. That is, a so-called three-beam system tracking control cannot be carried out.